“…Results for uniqueness are obtained by A. Aziz and M. Schneider [2], but up to now not a single example of nontrivial solution of the new problem, neither a general existence result is known. Some different statements of Darboux type problems in R 3 or some connected with them Protter problem for mixed type equations are investigated by A.Bitsadze [4], J. Barros-Neto and I. Gelfand [3], D. Lupo, C. Morawetz and K. Payne [11], D. Lupo, K. Payne, N. Popivanov [12], T. E. Moiseev [13], J. Rasiass [18], D. Edmunds, N. Popivanov [5], B. Keyfitz, A. Tesdall, K. Payne, N. Popivanov [19].…”